The bright sunshine of the tropics is great news for the group of microscopic plants called algae. They prosper, especially in shallow waters, and because they photosynthesize, they produce sugars that attract herbivores to feast on them. Many species of algae grow as a covering on underwater substrates, but others, such as diatoms and Volvox sp., live freely in the water column. Fishes can eat both types and some have become highly specialized to reap this rich harvest.

Can fish ever take advantage of the algal blooms that form in some aquatic habitats?

Pond owners are only too well aware that under certain conditions, especially in the summer months, their pools can turn into green soups. The organisms behind this phenomenon are algae, or more correctly, phytoplankton. Millions upon millions of these microscopic, single-celled plants contribute to this bloom yet very few fish are capable of taking direct advantage of it.

Nonetheless, the algae, which are at the bottom of the food chain, feed animals such as Daphnia, whose consequent rocketing numbers add up to a boon for the fish. One group of fish that do take advantage of the phytoplankton are the Tilapmes. These cichlids collect the algae from the water using their finely meshed gillrakers and specially secreted mucus to trap and collect them, almost like a spider's web.

What are Rift Lake cichlids eating when they graze on the rocks?

The waters of Lake Malawi contain a large number of cichlid species that graze on the algae covering the rocks. Whether these species should really be regarded as herbivores is a matter for debate. The reason for this is that the film of algae is also home to a huge number of tiny creatures such as ostracods, fly larvae, and copepods. One survey estimated that as many as 300,000 of these animals could be found in 4 square miles (I m2) of algae-coated rock surface. This tasty mix of animal and vegetable matter, sometimes referred to as “aufwuchs” in the literature, is a superb and rapidly replenishing source of food for a number of mbuna.

One species that specializes on this food is the cichlid Labeotropheus fuelleborm. Its distinguishing feature is a fleshy snout, which it is thought may act as a lever when the fish is cropping hair algae from the rocks. The ventrally situated mouth allows it to feed parallel to the rocks in extremely shallow waters, which are out of range for those fish with terminal mouths.

They have to stand on end to bite at the algae, and have lines of teeth inside the mouth that act like a file to rasp the aufwuchs from the rocks.

Do many fish eat plant leaves, or do they just graze their surfaces?

Some haplochromine cichlids take the algae from the plants directly, cleaning the leaves without damaging them. Other species of fish pass up the algae and instead feed on the plants themselves. The problem with this is that plants are generally tougher than algae so the fish have powerful jaws with slicing teeth that bite pieces out of the plant. Silver dollars (Metynnis spp. — not cichlids, but characins) specialize in this and can reduce aquatic plants to a few sorry shreds in record time. A school of these in the home aquarium is a beautiful sight but your aquascaping options are restricted to plastic plants.